There is a growing demand for achieving running of rolling stocks at a higher speed with higher acceleration/deceleration. Materialization of increased capacity and down-sized lighter-weight structure is constantly demanded for main motors. Greater capacity and down-sized lighter-weight structure of main motors have been promoted by simplification of structure, improvement of performance characteristic of magnetic material and also via inprovement of thermal and voltage resistant characteristics of insulation materials. In particular, materialization of greater capacity and down-sized lighter weight structures subsequent to debut of truck-mounted main motors replacing suspension-type main motors owes much to development of insulation technology. A variety of insulation materials exhibiting distinguished characteristics have been developed in recent years, and based on utilization of these insulation materials, further increase of capacity and further down-sizing and weight reduction have been promoted. For example, motor torque has drastically been improved since 1970 as a result of the introduction of H-level insulation subsequent to the utilization of polyimide resins exerting ultra-high resistance against heat as an insulation material.
Not only outstanding thermal resistant property, but stable mechanical and electrical characteristics in a wide range of temperatures are also demanded for those insulation materials cited above. Concretely, tensile strength relative to thermal degradation characteristic and dielectric breakdown strength relative to thermal degradation characteristic are respectively subject to evaluation. In addition, corona resistant characteristic is also demanded for insulation materials. In particular, corona resistant characteristic has more critically been considered in relation to introduction of alternate current for driving motors of electric trains in recent years.
Introduction of AC motors has been under study on the grounds that dimension can be contracted, compared with that of DC motors and the number of rotation can be increased to result in increased torque, and yet, maintenance can easily be performed. To compose insulated wires and coils, debut of such a film incorporating distinguished thermal resistant property, stable mechanical and electric characteristics, and distinguished corona resistant characteristic has been anticipated.
Polyimide film has proven to be satisfactory not only in thermal resistant property, but also in mechanical and electric characteristics, and thus, it can suitably be used as on insulation material of main motors. Nevertheless, polyimide film has not always proven to be satisfactory in corona resistant characteristic. Consequently, a variety of means have been executed to improve corona resistant characteristic of polyimide film.
In order to improve corona resistant characteristic of polyimide film, for example, a variety of methods such as adhesion of aggregate mica to polyimide film or dispersion of filler material such as mica, alumina or silica, in polyimide film have been implemented.
Nevertheless, because of the expensive cost of aggregate mica and the low reliability of adhesive agents used to adhere aggregate mica onto plastic film, and further, due to increased processing steps, production of such an insulation film comprising a plastic film adhered with aggregate mica results in the increased cost.
In the case of dispersing filler material in a plastic film, because of the complicated steps in changing kinds of filler in the process of producing film, product cost increases. Concretely, in the case of using polyimide film as a plastic film, for example, as shown in FIG. 9, the production process begins with synthesis and polymerization of varnish as a precursor of polyimide by conjunctionally feeding raw material of varnish and filler material to a reaction bath 1, and then, said varnish is led to an intermediate varnish tank 2, followed by a step of mixing said varnish from the intermediate tank 2 with reactive curing agent in a mixer 3 before eventually executing a film casting process, thus requiring very long processing steps. As was described above, since very long steps must be executed until reaching a final film casting process after synthesizing and polymerizing said varnish as a precursor of polyimide, whenever changing kind of filler to be added, tremendous mechanical loss arises from washing of facilities or the like. This in turn results in an increase in the cost of produced film to raise a critical problem in the case of using polyimide film as an insulation material.
Further, adding an increased amount of filler to the film results in improved corona resistant characteristic. However, this in turn raises such a problem as the mechanical strength of the film being lowered. Although addable amount varies to some extent depending on the kind of filler, in the case of adding alumina having mean particle size of scores of nm, the addable amount is limited to be about 20% by weight. In consequence, any conventional method which involves the addition of filler fails to drastically improve the corona resistant characteristic of film.
As preceding arts, Japanese Laid-Open Patent Publications SHO-50-665343 (1975) and HEI-4-122783 (1992) respectively proposed a method of improving the thermal radiative characteristic or corona resistant characteristic by providing a coated substance having 15.about.20 .mu.m of thickness and satisfactory thermal radiative characteristic. Nevertheless, according to the thickness of the coated substance used for implementing the above arts, in the case of manufacturing tape-wrapped insulated wires or insulated coils by applying the above-described coated film, because of substantial thickness of the film, when the coated film is wound on each wire, overall size of the insulated wire or the insulated coil expands. Since it is essential from the viewpoint of facilitating design work that usable film be thinner and fully satisfactory in electric characteristics including corona resistant characteristic for use in the latest compact and high-output motors, practical use of the above-cited film remains quite difficult.
Accordingly, in order to fully solve the above referred problems by way of drastically improving the above preceding arts from the standpoint of the existing industrial demand, the inventors have achieved the present invention as a result of following up overall studies and experiments. Concretely, the inventors have consummated a novel film capable of sustaining proper mechanical strength and insulating characteristic of plastic film such as polyimide film which is base material for insulating material, where the novel film is provided with distinguished corona resistant characteristic without significantly varying proper thickness of the base material. Accordingly, by way of utilizing the novel film, it is possible to provide such a film material suited for materializing compact-size and high-output potential required for the latest insulation material at an inexpensive cost. Furthermore, the invention also provides improved insulated wires, insulated coils, and electric motors, respectively being capable of responding to recent demand for materializing higher running speed and higher accelerating/decelerating function of rolling stocks.